Ducts and supply lines are often used in environments that are prone to contaminants such as moisture, dust, insects and gasses. Some form of seal therefore has to be provided either around the substrate (for example in the case of a supply line) or between two substrates (for example in the case of a duct seal between the duct and a supply line it carries).
Various techniques have been used in the past for forming such seals. The simplest perhaps is a tape wrap which has the advantage that a single product could be used over virtually any size or shape of substrate, but the disadvantage of poor performance. Sealing materials such as adhesives or mastics may be used alone or with tapes, but again performance is poor.
In recent years dimensionally-recoverable articles have been used for sealing supply lines and as duct seals. In general, a dimensionally-recoverable article is an article the dimensional configuration of which may be made substantially to change when subjected to a suitable treatment, for example heating. Usually these articles recover towards an original shape from which they have previously been deformed but the term also includes an article which adopts a new configuration even if it has not previously been deformed.
In their most common form such articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2027962, 3086242 and 3597372. In the production of heat-recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability.
An advantage of these articles is that they are strong, impermeable, water resistant and well able to resist such agencies as the small mechanical stresses caused by temperature variation, earth movements and repair work. They can be easily installed in their recoverable configuration and then caused to shrink (or otherwise recover) into sealing engagement with the substrate that they are to protect. In this way, ease of installation can be combined with excellent performance. A single size of product may be used with a variety of sizes of substrate, and close tolerances in manufacture may be avoided.
A difficulty, however, remains in some circumstances. The very fact that these articles recover on heating means that in general heat must be applied when the article is in approximately the desired position with respect to the substrate. This may be a problem where the substrate or some article nearby is a heat sink, where it physically prevents or hinders proper positioning of a torch or other heating means, where it is hazardous for example a gas pipe, or where it may be damaged by excessive heat.